The present invention relates to an apparatus for the application of a liquid or pasty medium, particularly a coating medium, onto a moving material web, in particular consisting of paper or board.
The apparatus includes at least one application unit for applying the liquid or pasty medium and a roll associated with the application unit wherein the roll to take up the liquid or pasty medium in the case of indirect application to the web, or the roll guides the material web past the application unit in the case of direct application of the medium to the web. The application unit has an application doctor blade which doctors the applied medium after it passes by the coating medium application chamber and a cleaning doctor blade which is arranged at a distance from the application doctor blade in the direction opposite the direction of rotation of the roll. The cleaning doctor blade together with the application doctor blade define boundaries of the application chamber for the liquid or pasty medium. Openings or channels are provided through which the liquid or pasty medium is discharged from the application chamber, essentially against the direction of the rotation of the roll. The liquid or pasty medium impinges at least partially outside the application chamber upon the surface region of the roll or material web, respectively, adjacent the cleaning doctor blade.
By means of apparatus; of this type, a liquid or pasty medium, particularly a coating medium, is applied in the form of color, impregnating liquid, starch, or the like onto the material web. The material web can consist of paper, or board or even of a textile material. The liquid or pasty medium can be applied onto the moving material web either directly or indirectly. In the case of direct application, the material web, lying on the shell surface of the roll, is passed by the application unit. The liquid or pasty medium is there applied directly on the web by the application doctor blade in the region of the application chamber and is applied onto the material web in the form of a coating film. The roll there serves as a counter-roll for the application unit. On the contrary, in indirect application on the web, the liquid or pasty medium is initially applied onto the surface of the roll and is subsequently taken up by the material web which is led past and in contact with the roll.
For achieving optimal quality both in direct and indirect applications, it is necessary that an absolutely uniform coating film be produced on the material web. However, the absolutely uniform formation of the coating film is prevented in practice if web components such as fibers, foreign bodies or dirt particles collect between the application doctor blade which is at the downstream or exit end of the application chamber and the roll surface of the web surface. In such cases, so-called blade streaks occur, which inevitably lead to a reduction in quality of the coating on the web.
In order to clean either the roll surface (indirect application) or the material web surface (direct application), which is to be coated, of such fibers, foreign bodies, dirt or like particles, it has already been suggested to provide a so-called cleaning doctor blade at a distance from the application doctor blade and in a direction opposite to the direction of rotation of the roll, that is, upstream of the application chamber. The particles should be scraped off by the cleaning doctor blade so that blade streaks are prevented upon application of the liquid or pasty medium.
An apparatus with an upstream cleaning doctor blade is already known, for example, from EP 0 319 503. A doctor blade knife is used as the cleaning doctor blade. In the direct vicinity of its effective surface or edge, this blade knife additionally has openings or channels through which the liquid or pasty medium flows out of the application chamber. The arrangement of the openings or channels in the blade knife is such that the discharging liquid or pasty flow of the medium impinges upon the roll or material web surface and rinses away the scraped off particles which may have gathered on that surface. In addition, this rinsing pre-wets the web which improves the quality of the subsequent application of coating.
In practice, relatively good rinsing and pre-wetting effect is achieved by means of this apparatus. However, under certain conditions, particles still collect upstream of or before reaching the cleaning blade knife. These mix with a thickened residual film and air bubbles under unfavorable conditions, and this mixture can build up in this form on the blade knife. In these cases, both the blade knife and the roll are subjected to uneven wear and also produce considerable coating film faults. Additionally, the scraped off particles which build up at the blade knife are taken up for a time by the roll and are pulled under the application blade rod downstream of the application unit, which leads to even greater film faults.